Method for supplying washing liquid to a process for cooking cellulose pulp

ABSTRACT

The invention relates to a method for supplying washing liquid to a digester ( 1 ) for cellulose pulp and a subsequent diffuser washing device ( 2 a,  2 b). A high, disturbance-free production capacity can be obtained by using a common feed pump (P 1 ) which, via a main feed line ( 10 ), supplies both the diffuser washing device and the digester with washing liquid in parallel. The first pump (P 1 ) establishes a pressure on the washing liquid which is in the interval 2-6 bar, and is preferably 4 bar, while the second pressure level, which is established by the diffuser pump device (P 2 a, P 2 b), is in the interval 6-12 bar, but is at least 2 bar higher than the first pressure level, and the third pressure level, which is established by the digester pump (P 3 ), is in the interval 12-25 bar. The system becomes self-regulating in the event of any operational disturbances in the flow of washing liquid and, if the pumps are appropriate, a countercurrent flow of washing liquid through the system is essentially maintained.

[0001] The present invention relates to a method for supplying washingliquid to a digester for cellulose pulp and a subsequent diffuser washerin accordance with the preamble to claim 1.

STATE OF THE ART

[0002] The combination of a continuous digester and a subsequentdiffuser is frequently employed when manufacturing chemical cellulosepulp, especially kraft pulp. As is customary, washing liquids for thedifferent constituent processes are conducted in countercurrent to theflow of the pulp, with the washing liquid first being fed to thediffuser, after which the wash filtrate which is obtained from thediffuser is conducted to the washing zone of the digester as dilutionliquid and/or washing liquid. This countercurrent flow of washingliquids is implemented for the purpose of reducing, to the greatestpossible extent, the consumption of fresh water, something which hasbeen regarded as being an absolute necessity for being able to close theprocess and minimize the discharge of process water from the process.U.S. Pat. No. 4,123,318 discloses such a countercurrent conveyance ofwashing liquid, with filtrate from a washing stage subsequent to thedigester being conducted to the preceding washing stage in the processand from the washing stage to the washing zone of the precedingdigester. U.S. Pat. No. 5,066,362 also discloses a system in which thewashing liquid is firstly conducted to a diffuser after which the washfiltrate which is obtained from the diffuser is conducted to the washingzone of the digester. This provides a gradual leaching-out/washing ofthe pulp and a gradually increasing content of released material in thewash filtrate which is being conducted in countercurrent to the flow ofthe pulp. The washing liquid is also conducted strictly incountercurrent to the flow of the pulp in SE,C,501848, with the washingliquid firstly being conducted to a diffuser, after which the diffuserfiltrate is conducted to the washing zone of the digester. All thatotherwise takes place is that a supplement of white liquor is conveyedto the diffuser filtrate for the purpose of increasing the level ofalkali in the diffuser filtrate.

[0003] In order to increase the operability in these systems, buffertanks or intermediate storage tanks are used for the diffuser filtrateso as to ensure that washing liquid can be supplied to the digester at aconstant rate. These tanks frequently have to be designed as expensivepressure vessels since the temperature of the washing liquids at thesepositions is close to, or above, the boiling point of the washingliquid.

[0004] The known systems function satisfactorily when production ismoderate and if there are no disturbances in the process. However, ifthe diffuser, whose washing efficiency is normally very good, shouldexhibit a tendency to cause clogging in the flow of washing liquids,due, for example, to inappropriate handling, this then affects thedigester directly as a result of a corresponding decline in washingliquid and dilution liquid being supplied to the bottom of the digester.While this greatly reduces the washing effect at the bottom of thedigester, another marked effect is that it can become difficult toachieve the dilution in pulp concentration which is required inassociation with the pulp being blown from the digester, something whichimmediately involves the risk of giving rise to clogging in the flow ofpulp from the digester and of the temperature of the pulp rising rapidlyto an undesirable level. In order to deal with this clogging, it may benecessary for the digester to be shut down completely.

OBJECT AND PURPOSE OF THE INVENTION

[0005] The principal object of the invention is to avoid thedisadvantages of the prior art and to enable the operability of thecooking process to be improved in the digester/diffuser constituentsystem, with it being possible to increase the productivity in anexisting digester (more pulp per 24 hours) by more than 100%. By meansof totally integrating the digester and the diffuser, most of theextraction from the cooking can instead be transferred to the diffuserand the loading on the digester's extraction screens can consequently bereduced, thereby making it possible to achieve this substantialupgrading of the digester capacity. This becomes particularlyadvantageous when upgrading existing digesters. As a result of a largepart of the extraction from the cooking being made in the pressurediffuser, the pressure and/or liquor level in the digester can becontrolled by means of this extraction from the diffuser.

[0006] Another object is to make the process less sensitive todisturbances in the flow of washing liquid through the diffusers, withit being possible to maintain the digester process under optimumconditions despite the occurrence of momentary disturbances in the flowof washing liquid through a diffuser.

[0007] Yet another object is to optimize the procedure with regard tosupplying washing liquid such that only that pressure level which isrequired is maintained at each point in the supply system, therebymaking it possible to set the greater part of the system at a moderatelylow level, thus enabling thinner pipelines to be used.

[0008] Yet another object is to obtain a system for providing thediffuser and digester with washing liquid, which system isself-regulating and can at any moment guarantee the requisite flow ofwashing liquid to the washing and dilution zone of the digester. Pulpcan then be guaranteed to be fed out from the digester at the correctconsistency and temperature, thereby actively counteracting any tendencytowards clogging in the pulp flow leaving the digester, on the one hand,and, on the other hand, eliminating any lowering in the quality of thepulp due to the blowing temperature being too high. As a result of thisself-regulation, the major part of the flow of washing liquid to thebottom of the digester will pass via the diffusers during normaloperation. However, as soon as any local blockage of washing liquidoccurs in a diffuser, a self-regulation then takes place, with thisself-regulation compensating for the decline in filtrate flow from thediffuser with an equivalent flow from the diffuser's washing liquidtank.

[0009] Yet another object is to be able to remove all the buffer orintermediate storage tanks for the diffuser filtrate, therebysubstantially reducing the investment costs.

LIST OF DRAWINGS

[0010]FIG. 1 shows a diagram of a system according to the invention,with a digester and two subsequent diffusers which are coupled inparallel:

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0011]FIG. 1 shows a system according to the invention for supplyingwashing liquid to a continuous digester 1 for cellulose pulp and asubsequent diffuser washing device 2 a, 2 b in which washing liquid 12a, 12 b for the diffuser washing device is drawn from a storage tank 4and in which washing liquid 17 is conducted to the bottom of thedigester in order to obtain a dilution and/or a displacement washing atthe bottom of the digester.

[0012] The cellulose pulp which is dissolved in the digester is fed outin the flow 20 and a Y-coupling arrangement is preferably used to ensurethe establishment of two constituent flows 20 a, 20 b to two diffusers,which consequently operate in parallel. The central branch in the Ycoupling provides an alternative auxiliary feeding route for the flow 20a or else the flow 2Ob. After the diffusers, washed pulp 21 a, 21 b iswithdrawn from each respective diffuser and combined in the flow 22 forsubsequent conduction to a storage tank 3 for the cooked and washedpulp.

[0013] The diffuser is a conventional pressure diffuser which maintainsthe pressure in the pulp during the washing. The diffuser can be eitherfed from the bottom or the top, and the figure shows a top-fed pressurediffuser. The pulp is conducted into the top of the pressure diffuser,after which it passes down through the pressure diffuser in a thincircular layer, with the washing liquid being fed in from outside thelayer (marked grey in the figure) by way of a multiplicity ofdistribution nozzles 14 a, 14 b, which are arranged, on the one hand,around the whole of the pressure diffuser (not shown) and, on the otherhand, over the greater part of the height of the pressure diffuser (asshown in the figure). The washing filtrate which is obtained after thewashing liquid has passed through the fibre layer is extracted from theinside of the pressure diffuser, 15 a, 15 b, with these filtrate flowsbeing indicated by 13 a and 13 b in the figure.

[0014] That which is characteristic of the invention is that the washingliquid from the storage tank 4 is pressurized to a first pressure level,by way of a first feeding pump P1, and is subsequently conducted to amain feed line 10. At a first upstream point 10 a in the main feed line,a first constituent flow 11 is drawn from this main feed line 10 to adiffuser pump device P2 a/P2 b which pressurizes the washing liquid to asecond pressure level and conducts it onward, 12 a, 12 b, at this secondpressure level to the washing liquid inlets 14 a, 14 b belonging to adiffuser washer. The wash filtrate 13 a, 13 b from the washing liquidoutlet 15 a, 15 b in the diffuser washer is conducted back to the mainfeed line 10 at a second point 10 b in the main feed line, with thissecond point being located downstream of the first upstream point 10 ain the main feed line. A combined flow of washing liquid from thestorage tank 4 and wash filtrate 16 from the diffuser washing device 2a, 2 b is conducted, via the main feed line, to a digester pump P3,which pressurizes the combined flow to a third pressure level, and isconducted onwards 17, at this third pressure level, to washing liquidnozzles 18 which are arranged at the bottom of the digester.

[0015] The pressure levels are related to each other such that the firstpressure level is lower than the second pressure level, which secondpressure level is in turn lower than the third pressure level, with eachrespective pressure level constituting an increase in pressure of atleast 50-125% relative to the next lower pressure level.

[0016] In a preferred application, this corresponds to the firstpressure level, which is established by the first feed pump P1, being inthe interval 2-6 bar, preferably 4 bar, to the second pressure level,which is established by the diffuser pump device P2 a, P2 b, being inthe interval 6-12 bar, but at least 2 bar above the first pressurelevel, and to the third pressure level, which is established by thedigester pump P3, being in the interval 15-25 bar.

[0017] In a conventional pressure diffuser, the pump P2 a should delivera pressure which guarantees flow through the diffuser. The followingfalls in pressure occur in the diffuser circuit:

[0018] a) static height (difference in height between the pump and theupper part of the diffuser, approx. 2.5 bar);

[0019] b) pressure fall in lines and control valves (FC etc., approx. 1bar);

[0020] c) the diffuser's inlet nozzles (14 a, approx. 2 bar);

[0021] d) pressure fall through the pulp bed (approx. 0.5 bar);

[0022] e) prevailing pulp pressure at the top of the diffuser (dependson the digester pressure, its height + pressurization level, cantypically be 6 bar).

[0023] With a feed pump P1 which delivers 4 bar, a guaranteed flow undernormal operation is obtained if P2 a provides an increase in pressure ofa good 8 bar, on the assumption that the pulp pressure at the top of thediffuser is 6 bar. The governing prerequisite is that, under normaloperation, the pressure on the flow 16 should at least be equal to thepressure of the feed pump.

[0024] The connection points 10 a and 10 b should expediently be locatedon the main feed line such that the point 10 b lies at a distance fromthe digester pump P3 which is within 10% of the entire length of thefeed line 10 and such that the point 10 a lies at a distance from theoutlet of the feed pump P1 which is within 10% of the entire length ofthe feed line 10. During normal, disturbance-free operation in thesystem, the fall in pressure in the feed line 10 between the points 10 aand 10 b should expediently exceed or correspond to the increasedpressure in the washing liquid filtrate 13 a, 13 b.

[0025] In the embodiment shown, the diffuser washing device consists oftwo diffusers which are coupled in parallel as regards the receipt andtreatment of cooked pulp obtained from the digester. This configurationprovides increased availability in the system, with it being possible toredirect part of the pulp flow between diffusers depending on anypossible momentary disturbances in the system.

[0026] The diffuser pump arrangement consists of two diffuser pumps P2 aand P2 b, which are coupled in parallel and which, on the inlet side,are connected to the same supply flow 11, which is taken from the mainfeed line 10, with each respective outlet from each of the diffuserpumps only supplying one diffuser with its respective washing liquidflow 12 a, 12 b. While P2 a and P2 b can, of course, be replaced with asingle pump, two separate pumps provide increased availability. The washfiltrates 13 a and 13 b, which are obtained from each respectivediffuser, are first of all combined to form a joint flow 16 in a Ycoupling,. after which this combined wash filtrate flow 16 is returnedto the main feed line at a point 10 b on the main feed line closer tothe inlet to the digester pump P3.

[0027] The whole of the system becomes self-adjusting with regard to anypossible variations in the pulp flow and/or washing liquid flows, and nobuffer tanks or intermediate storage tanks are required. When the pumpsare adequate, the main flow of the washing liquid filtrate develops inthe circuit 4-P1-11-P2 a/P2 b-2 a/2 b-16-P3-1; however, as soon asmomentary restrictions in flow occur in the circuit due to pressurefalls in the diffuser which are too great, the decline in the flow isthen compensated by flow in the circuit P1-10 a-10-10 b-P3. In anapplication in which the capacity in an existing digester was increasedfrom 1 500 AdT/24 hours to somewhat more than double the capacity, i.e.3 200 Adt/24 hours, this then means that the rate of flow of the chipsin the digester was more than doubled, representing a dramatic increasein production capacity. In connection with flow rates of this magnitudein the system, great demands are made on the retention of a high levelof supply of washing liquid and dilution liquid to the bottom of thedigester, thereby guaranteeing correct dilution and washing prior to thepulp being blown from the digester. The invention allows the existingproduction capacity to be increased dramatically while at the same timemaintaining a high degree of operability and availability.

[0028] The invention can be varied in a number of ways within the scopeof the attached patent claims.

[0029] For example, the parallel diffusers can be replaced by a singlediffuser or, alternatively, supplemented with a third diffuser which iscoupled in parallel.

[0030] The embodiment depicted in FIG. 1 shows flow-controlling valvesFC which act on flows 12 a and 12 b and also on the flow 17. The flow 17is normally divided up into several constituent flows which are added tothe bottom of the digester at different heights.

[0031] The diffuser/diffusers 2 a, 2 b can also have one or morediffusers which are coupled in series, for example such that the pulpflow either passes by way of 2 a ₁-2 a ₂-2 a ₃ or, alternatively, by wayof 2 b ₁-2 b ₂-2 b ₃, where the subscript ₁/₂/₃ indicates the order inthe direction of flow of the pulp. A first joint feed pump can thenprovide all the diffusers in at least one main feed line withpressurized washing liquid from a diffuser tank, which branch is in opencommunication with washing/dilution nozzles in the bottom of thedigester, with the wash filtrate from each respective diffuser beingreturned to the main feed line. The pump P2 ₃ which supplies the lastdiffuser is designed/selected such that the increase in pressurerelative to the pressure in the main feed line exceeds the pressure fallin the diffuser and the prevailing pulp pressure at that position. Whenthe diffusers are identical, a gradual increase in pressure in thedirection P2 ₃→P2 ₂→P2 ₁, on a par with 2 bar, should normally beimplemented. In a typical example in which the first feed pump provides4 bar, the pressure levels P2 ₃/P2 ₂/P2 ₁ can be on a par with ₃/₅/₇ barif the prevailing pulp pressure in each respective diffuser (given bythe pressure in the preceding digester) is on a par with ₄/₆/₈ bar.

[0032] Sometimes, use is made of two or more parallel pumps whichreplace the pump P3, with each of the respective pumps supplying its ownconstituent flow to the bottom of the digester. Flow-controlling valvesFC are also arranged in the Y coupling which regulates the flow of pulpto the parallel diffusers. Pressure-controlling outlet valves PC arealso arranged in the outlets.

1. Method for supplying washing liquid to a digester (1) for cellulosepulp and a subsequent diffuser washing device (2 a, 2 b), with washingliquid for the diffuser wash being collected from a storage tank (4) andwith washing liquid being conducted to the bottom of the digester forthe purpose of obtaining a dilution and/or displacement wash at thebottom of the digester, characterized in that the washing liquid fromthe storage tank (4) is pressurized to a first pressure level via afirst feed pump (P1) and conducted onwards to a main feed line (10), afirst constituent flow (11) is drawn from the main feed line (10), at afirst upstream point (10 a) on the main feed line, to a diffuser pumpdevice (P2 a, P2 b), which pressurizes the washing liquid to a secondpressure level, and is conducted onwards (12 a, 12 b), at this secondpressure level, to the washing liquid inlets (14 a, 14 b) of a diffuserwasher, wash filtrate from the washing liquid outlet (15 a, 15 b) of thediffuser washer (2 a, 2 b) is returned to the main feed line (10) at asecond point (10 b) on the main feed line, which point is locateddownstream of the first upstream point (10 a) on the main feed line, acombined flow of washing liquid from the storage tank (4) and washfiltrate (16) from the diffuser washing device (2 a, 2 b) is conducted,via the main feed line (10), to a digester pump (P3) which pressurizesthe combined flow to a third pressure level and conducts this flow (17)onwards, at this third pressure level, to washing liquid nozzles (18)which are arranged at the bottom of the digester.
 2. Method according toclaim 1, characterized in that the first pressure level is lower thanthe second pressure level, which second pressure level is in turn lowerthan the third pressure level.
 3. Method according to claim 2,characterized in that each respective pressure level constitutes anincrease in pressure of at least 50-125% as compared with the next lowerpressure level.
 4. Method according to claim 3, characterized in thatthe first pressure level, established by the first feed pump (P1), is inthe interval 2-6 bar, and is preferably 4 bar, in that the secondpressure level, established by the diffuser pump device (P2 a, P2 b), isin the interval 6-12 bar, but is at least 2 bar higher than the firstpressure level, and in that the third pressure level, established by thedigester pump (P3), is in the interval 12-25 bar.
 5. Method according toany one of the preceding claims, characterized in that the diffuserwashing device (2 a, 2 b) consists of at least one pressure diffuser,which maintains a positive pressure in the pulp during the washing. 6.Method according to any one of the preceding claims, characterized inthat the diffuser washing device consists of two diffusers (2 a, 2 b),which are coupled in parallel with regard to the receipt and treatmentof cooked pulp (20) obtained from the digester.
 7. Method according toclaim 6, characterized in that the diffuser pump device consists of twodiffuser pumps (P2 a, P2 b) which are coupled in parallel and which areconnected, on the inlet side, to the same supply flow (11) taken fromthe feed line (10), with the respective outlet from each of the diffuserpumps only supplying one diffuser.
 8. Method according to claim 7,characterized in that the wash filtrate (13 a, 13 b) which is obtainedfrom each respective diffuser is firstly combined into a joint flow (16)in a Y coupling, after which this combined wash filtrate flow isreturned to the main feed line (10) at a point (10 b) on the main feedline which is closer to the inlet of the digester pump (P3).
 9. Methodaccording to any one of the preceding claims, characterized in that thesystem for supplying the washing liquid to the digester and diffuserlacks any buffer or intermediate storage tanks after the first feed pump(P1) has pressurized the washing liquid.